1. Field of Invention
The present invention relates to a method for measuring greenhouse gases using an infrared absorption spectrometer. More particularly, the present invention relates to a method for measuring greenhouse gases using a Fourier-transform infrared spectroscope (FT-IR).
2. Description of Related Art
In recent years, problems relating to global warming are attracting more attention while greater concerns in environmental problems being raised. Global warming progresses in association with increases in the concentration of greenhouse effect gases (or greenhouse gases) in the atmosphere, such as CO2, NOx, methane, PFC (perfluorocarbon) and the like. These greenhouse gases have strong absorbance in an infrared region. When discharged into the atmosphere, they absorb energy irradiated from the earth""s surface. The absorbed energy is radiated upward toward outer space and downward toward the earth""s surface. In this instance, a part of the energy radiated from the earth""s surface is returned again to the earth""s surface by the greenhouse gases, such that the temperature on the earth""s surface increases. By this system, the greenhouse gases are thought to bring about a global warming effect.
Global warming potential (GWP) is known as an indicator to compare degrees of the global warming effect caused by the greenhouse gases. The GWP represents how much warming effect one unit weight of a gas has compared to one unit of CO2. PFCs are counted as gases having a high global warming effect among the greenhouse gases. PFCs have an extremely high GWP value. For example, the GWP value of CF4 is about 6,500 times higher than that of CO2. Also, PFC is stable compared to other gases, and have a very long life in the atmosphere. For example, the life of CF4 in the atmosphere is about 50,000 years. Therefore, once discharged into the atmosphere, PFC would warm the earth for many years.
The PFCs are ordinarily used in a process of manufacturing semiconductor devices, and more particularly, they are frequently used in an apparatus using low-pressure plasma. For example, in a dry etching apparatus, PFCs, such as CF4, C4F8 and the like, are used to etch SiO2 and Si3N4. Also, in a CVD apparatus, gas plasma of C2F6 or the like is often used to clean films of silicon compound or the like adhered to the apparatus. Furthermore, a liquid PFC is used as a medium to cool wafers. However, as described above, since the PFC has a high global warming effect, the reduction in the discharged amount of PFCs is purposefully being pursued. At the COP3 Kyoto Conference that was held in December 1997, an agreement was reached to reduce the discharged amount of PFCs and the like in Japan by 6% by the year 2010 compared to the level in the year 1995. Thereafter, at the ESH (environmentxc2x7safetyxc2x7health) task force in the WSC (World Semiconductor Conference) held in 1999, an agreement was reached to reduce the total discharged amount by 10% by the year 2010 compared to the level in the year 1995.
In order to verify the reduction of the discharged amount of PFCs, PFC gases that are emitted from factories need to be measured. At present, since it is difficult to measure PFC gases emitted from a factory, PFC gases discharged from a semiconductor manufacturing apparatus that uses PFCs are measured, and the discharged amount of PFCs is determined using an amount ratio (emission factor) of discharged gases to fed gases, and the amount of gases that are consumed in the factory. PFC gases that are actually discharged from a semiconductor manufacturing apparatus are measured to calculate the emission factor. Measurements of PFC gases that are actually discharged from semiconductor manufacturing apparatuses are conducted according to guidelines referred to as xe2x80x9cEmissions Characterization Package Rev. 2.4 (generally referred to as xe2x80x9cIntel Protocolxe2x80x9d) (currently, xe2x80x9cEquipment Environmental Characterization Guidelines Rev. 3.0xe2x80x9d) by J. Mayers et al. of Intel Corporation, which was distributed at Global Semiconductor Industry Conference on Perfluorocompound Emissions Control that was held in Monterey, Calif., U.S. in April 1998. The Intel Protocol describes a method for measuring emitted PFC gases, using a quadrupole mass analyzing spectrometer (QMAS) and a Fourier-transform infrared spectroscope (FT-IR). However, since no detailed description is provided for the measurements using the FT-IR, measurement results may have great variations depending on measurement methods. Accordingly, to compensate for the Intel Protocol, methods of measuring PFCs in emitted gasses with simpler processes and good reproducibility have been sought.
It is an object of the present invention to provide a method for measuring components in emitted gases with ease and good reproducibility, using an infrared absorption spectrometer.
(A) A method for measuring greenhouse gases using an infrared absorption spectrometer in accordance with the present invention includes the steps of:
selecting a process chemical material;
selecting a measurement target chemical material corresponding to the process chemical material:
designating expected concentration ranges for the process chemical material and the measurement target chemical material;
selecting libraries for the respective expected concentration ranges for the process chemical material and the measurement target chemical material; and
analyzing data obtained by a gas infrared absorption spectrometry based on the libraries.
The process chemical material is a chemical material that is used in a process. Also, the measurement target chemical material is a chemical material that may possibly be included in the gas because the process chemical material is included in the gas.
Also, the library is absorbance data that is measured in advance for known concentration of the chemical material, which is used to perform an accurate concentration measurement, when data obtained by an infrared absorption spectrometry of the chemical material is analyzed.
By the method for measuring greenhouse gases in accordance with the present invention, concentrations of respective components in a mixed gas can be measured with a simplified method and with good accuracy.
For the method for measuring greenhouse gases in accordance with the present invention, the following description is provided of exemplary embodiments (1)xcx9c(3).
(1) A library formed of absorbance data for a plurality of known concentrations is made for each chemical material in the gas;
calibration curve data with respect to concentration-absorption area for a main peak region and an auxiliary peak region is made for the each chemical material in the gas based on the library; and
data obtained by the gas infrared absorption spectrometry is analyzed based on the calibration curve data.
When the expected concentration range of each composition among the gas is included in a region where a linearity of the calibration curve data for the main peak region is small, the concentration is determined using the calibration curve data for the auxiliary peak region.
Here, the xe2x80x9cmain peak regionxe2x80x9d is a region that includes the highest peak in the infrared absorption waveform of the gas. Also, the xe2x80x9cauxiliary peak regionxe2x80x9d is a region that includes a peak located in a region that is different from the main peak region. Depending on compositions, two or more auxiliary peak regions may exist.
Also, the xe2x80x9ccalibration curve dataxe2x80x9d is data representing the relation between concentrations and absorption areas for each component in the gas. When an infrared absorption measurement is conducted, libraries are selected in accordance with the expected concentration ranges, and data analysis is conducted based on calibration curve data for the selected libraries.
Also, the xe2x80x9cregion where a linearity of the calibration curve data for the main peak region is smallxe2x80x9d is an area in the calibration curve data for the main peak region that has a smaller linearity in the relation between concentrations and absorption areas, compared to the calibration curve data for the auxiliary peak region. By the method described above, the calibration curve data for the auxiliary peak region is used to determine the concentration described above, with the result that concentration of each of the components in the gas can be measured with high accuracy.
Alternatively, when the expected concentration range of each composition among the gas is included in a region where a linearity of the calibration curve data for the auxiliary peak region is small, the concentration is determined using the calibration curve data for the main peak region. Here, the xe2x80x9cregion where a linearity of the calibration curve data for the auxiliary peak region is smallxe2x80x9d is an area in the calibration curve data for the auxiliary peak region that has a small linearity in the relation between concentrations and absorption areas, compared to the calibration curve data for the main peak region.
By the method described above, the calibration curve data for the main peak region is used to determine the concentration described above, with the result that concentration of each of the components in the gas can be measured with high accuracy
Also, in this case, the concentration may be determined using both of the calibration curve data for the main peak region and the calibration curve data for the auxiliary peak region.
(2) The process chemical material may include at least one of CF4, CHF3, C2F4, C2F6, C3F8, C4F8, C5F8, HF, SiF4, NF3, SF6 and N2O.
(3) The measurement target chemical material may include at least one of CF4, CHF3, C2F4, C2F6, C3F8, C4F8, C5F8, COF2, HF, SiF4, OF2, NF3, SO2, SF6, SO2F2, SOF2, NO, N2O, NO2, CO and CO2.
(B) A method for measuring greenhouse gases using an infrared absorption spectrometer in accordance with another aspect of the present invention is a method in which, when data for gas obtained by an infrared absorption spectrometry is analyzed, a calibration curve representing absorption areas with respect to concentrations is made for each of a main peak region and an auxiliary peak region for each component in the gas. When a concentration of each composition among the gas is expected to be included in a region where a linearity of the calibration curve for the main peak region is small, the concentration is determined using the calibration curve data for the auxiliary peak region. By the method, the effects described above can be obtained.
(C) A method for measuring greenhouse gases using an infrared absorption spectrometer in accordance with yet another aspect of the present invention is a method in which, when data for gas obtained by an infrared absorption spectrometry is analyzed, a calibration curve representing absorption areas with respect to concentrations is made for each of a main peak region and an auxiliary peak region for each component in the gas. When a concentration of each composition among the gas is expected to be included in a region where a linearity of the calibration curve for the auxiliary peak region is small, the concentration is determined using the calibration curve for the main peak region. By the method, the effects described above can be obtained.
In either of methods (B) and (C) described above, the concentration may be determined using both of the calibration curve for the main peak region and the calibration curve for the auxiliary peak region.
Also, for a portion of the calibration curve having a low linearity, a correction to increase measurement points adjacent the portion may be performed with respect to the calibration curve.
Here, the xe2x80x9cmeasurement pointxe2x80x9d is data representing a value of an absorption area with respect to a specified concentration for each composition in the gas. The xe2x80x9cportion of the calibration curve having a low linearityxe2x80x9d is a portion that is deviated from an equation that represents a specified relation to be established between concentrations and absorption areas for each of the components in the gas. By performing the correction, the linearity of the calibration curve can be increased. As a result, by analyzing data based on the data with respect to the corrected calibration curve (calibration curve data), the concentration of components included in the gas can be accurately calculated.
Also, for a portion of the calibration curve having a high linearity, the calibration curve may be made using one measurement point or two measurement points in the portion having a high linearity. By the method, a highly accurate calibration curve can be made even with fewer measurement points.
(D) A method for measuring greenhouse gases using an infrared absorption spectrometer in accordance with yet another aspect of the present invention includes the steps of:
analyzing data based on an absorption waveform for a gas obtained by an infrared absorption spectrometry;
measuring components on priority basis, among components in the gas, having peaks that do not overlap peaks of the other components; and
successively subtracting the peaks of the components from the absorption waveform.
By the method, even when peaks concur with one another or overlap one another for each of the components, the concentration of each of the components can be accurately measured.
In this case, the following steps (a)xcx9c(j) can be performed in the method:
(a) subtracting peaks of NO and SO2F2 from the absorption waveform of the gas to measure NO and SO2F2;
(b) subtracting a peak of COF2 from the absorption waveform obtained in the step (a) to measure COF2;
(c) subtracting a peak of SOF2 from the absorption waveform obtained in the step (b) to measure SOF2;
(d) subtracting a peak of OF2 from the absorption waveform obtained in the step (c) to measure OF2;
(e) subtracting peaks of SF6, NF3 and C4F8 from the absorption waveform obtained in the step (d) to measure SF6, NF3 and C4F8, respectively;
(f) subtracting peaks of C3F8 and SiF4 from the absorption waveform obtained in the step (e) to measure C3F8 and SiF4, respectively;
(g) subtracting peaks of N2O, C2F6 and C2F4 from the absorption waveform obtained in the step (f) to measure N2O, C2F6 and C2F4, respectively;
(h) subtracting peaks of CF4, SO2and CO from the absorption waveform of the gas obtained in the step (g) to measure CF4, SO2and CO, respectively;
(i) measuring CHF3 from the absorption waveform obtained in the steps (a)xcx9c(h); and
(j) subtracting peaks of HF, CO2 and NO2 from any one of the absorption waveform of the gas and the absorption waveforms of the gas obtained in the steps (a)xcx9c(i).
Furthermore, in this case, in addition to the above steps (a)xcx9c(j), one or two or more of the following steps (1)xcx9c(8) may be performed.
(1) HF, CO2and NO2 are measured by any one of the steps (a)xcx9c(i) instead of the step (j), or by the step that is different from the steps (a)xcx9c(i) but is conducted before the step (j).
(2) SF6 is measured by any one of the steps (f)xcx9c(j) instead of the step (e), or by the step that is different from the steps (f)xcx9c(j) but is conducted after the step (e).
(3) CF4 is measured by the step (i) or the step (j) instead of the step (h), or by the step that is different from the step (i) or the step (j) but is conducted after the step (h).
(4) SiF4 is measured by any one of the steps (g)xcx9c(j) instead of the step (f), or by the step that is different from the steps (g)xcx9c(j) but is conducted after the step (f).
(5) CO is measured by the step (i) or the step (j) instead of the step (h), or by the step that is different from the step (i) or the step (j) but is conducted after the step (h).
(6) C2F6 is measured by any one of the steps (h)xcx9c(j) instead of the step (g), or by the step that is different from the steps (h)xcx9c(j) but is conducted after the step (g).
(7) C2F4 is measured by any one of the steps (h)xcx9c(j) instead of the step (g), or by the step that is different from the steps (h)xcx9c(j) but is conducted after the step (g).
(8) C4F8 is measured by the step (f) instead of the step (e), or by the step that is different from the step (f) but is conducted after the step (d) but before the step (g).
(E) In a method for measuring greenhouse gases using an infrared absorption spectrometer in accordance with yet another aspect of the present invention, when an infrared absorption spectrometry is conducted for a gas, one or more of components in the gas may be used as correction gases.
In the method described above, a plurality of values for concentrations of the correction gases to be used for correction may be set at equal intervals as viewed in logarithmic values of the concentrations. By the method, since the measurement points can be disposed at equal intervals on logarithmic scale, a highly accurate calibration curve can be made with fewer measurement points.
Also, when CF4 or SF6 is selected as the correction gas, a calibration curve representing absorption areas for concentrations with respect to a main peak region regarding the correction gas can be made, and a correction can be made at a portion having excellent linearity in the calibration curve with respect to the main peak region. By the method, a highly accurate correction can be conducted.
In accordance with the present invention, the gas that is subject to a measurement using an infrared absorption spectrometer may include at least one of CF4, CHF3, C2F4, C2F6, C3F8, C4F8, C5F8, COF2, HF, SiF4, OF2, NF3, SO2, SF6, SO2F2, SOF2, NO, N2O, NO2, CO and CO2.